A conventional tire pneumatic pressure monitoring system will be described with reference to FIG. 4. As shown in FIG. 4, sensors 11a to 11d are respectively provided in tire valves of respective tires 3a to 3d of a vehicle 2. Further, a monitoring system 12 is provided in the vehicle 2. The sensors 11a to 11d detect tire information, such as pneumatic pressure, temperature of the corresponding tires 3a to 3d, convert the tire information into radio signals and transmit the signals to the outside of the tires. In this case, the sensors 11a to 11d are designed to transmit the radio signals in different transmission patterns according to whether the pneumatic pressure and temperature of the tires 3a to 3d are normal or abnormal. Also, the respective sensors randomly intermittently transmit radio signals.
The monitoring system 12 has four receiving antennas 13a to 13d, a receiving set 14 and an indicator 15. The receiving set 14 has a receiving circuit 16 and a microcomputer 17. Also, the respective receiving antennas 13a to 13d are connected to the receiving circuit 16. The receiving circuit 16 demodulates radio signals into pulse signals and outputs them to the microcomputer 17. Also, a received signal strength indicator(RSSI) circuit is built in the receiving circuit 16, and the receiving circuit 16 outputs a received signal strength indicator signal (referred to as an RSSI signal) of radio signals received by the respective receiving antennas 13a to 13d to the microcomputer 17.
The microcomputer 17 comprises a CPU unit composed of CPU, ROM and RAM. Further, reference data such as pneumatic pressure and temperature of the tires 3a to 3d are stored in the microcomputer 17. The reference data indicates normal values of the pneumatic pressure and temperature of the tires 3a to 3d, and is set to be in a predetermined range. The indicator 15 is arranged within the vehicle 2, and indicates contents by a differential signal from the microcomputer 17 when abnormality occurs in the tires 3a to 3d. 
Further, detecting units 18a to 18d are provided in the conduction paths between the respective antennas 13a to 13d, and the receiving circuit 16. These detecting units 18a to 18d are designed to respectively control availability of the reception of the corresponding receiving antennas 13a to 13d based on the differential signal from the microcomputer 17 (see Japanese Unexamined Patent Application Publication No. 2003-11833 (FIG. 5)). The control of such a system is complicated because the microcomputer controls availability of the reception of the detecting units.